Thank you for all the suggestions! I've never tried the "call to phone" feature of Skype, but I hear it works great. It allows cheap outgoing calls and even allows incoming calls, but you have to pay extra for it. ooVoo sounds similar to Skype.
The best suggestion so far for my purposes is the Whistle Phone app, which "provides free ad-supported 20 minute inbound and outbound calls to and from your Whistle number". I just downloaded the app, and it's working great! Hopefully, I won't have any problems with it in the hotels in Europe. I'll post my experience with the app in this thread when I return.